1. Technical Field
The present disclosure relates to a wiring board having a shield function, and more particularly, to an inductor built-in wiring board having a shield function.
2. Related Art
In a power supply circuit used for a cellular phone, a personal computer, a portable terminal, a PDA, a vehicle control device, a small-size communication machine, a small-size game machine, and the like, inductors (switching power supply or the like) are essential components, and the inductors are separately mounted on a printed circuit board. That is, as a general method, electronic components or electronic circuit elements are mounted on the printed circuit board, and discrete coils are soldered onto the printed circuit board when components having desired circuit functions such as inductors are necessary.
FIG. 1 shows an inductor according to the related art. In FIG. 1, an inductor is formed by winding a coil 2 around a magnetic core 1 and is covered with a shield 3. The inductor is mounted on a printed circuit board (not shown) through mounting terminals 4. An electric current 5 is allowed to flow in the coil 2, thereby generating a magnetic flux 6.
According to the related-art method of mounding the components on the printed circuit board as described above, it is necessary to prepare the discrete coil, and thus management of components is complicated. In addition, there might be a problem in reliability for mounting components. Moreover, a considerable space for mounting components such as coils is necessary, and thus it is difficult to reduce a size of a device.
However, since a technique of forming a conductive pattern on a printed circuit board has been advanced, an attention is paid on a method of directly forming a coil on a printed circuit board with a conductive pattern. In the method of forming a coil on a printed circuit board with a circuit pattern, in order to obtain a desired inductance value, it is necessary to increase a size of the pattern. As a result, a size of the printed circuit board increases, and thus it is difficult to reduce the size of the device.
In order to solve such a problem, JP-A-5-183274 discloses a printed circuit board in which a coin requires no space for fixing, and whereby a desired inductance value can be obtained in a limited area on a printed circuit. Specifically, a printed circuit board formed of multi-layers includes a coil having a given wire width, wire distance, and number of windings. The coil is formed such that winding directions of adjacent spiral patterns are reverse directions to each other, and the spiral patterns are coupled by a blind via-hole, an inner via-hole, or a through-hole. As an example, a spiral pattern which is wound clockwise from a terminal part is formed on a first layer. The other end portion of the pattern of the first layer is coupled with a pattern of a second layer via a blind via-hole and the pattern is wound counterclockwise. The other end portion of the pattern of the second layer is coupled with a pattern of a third layer via a through-hole and the pattern is wound clockwise. The end portion of the pattern of the third layer is coupled with a pattern of a fourth layer via a blind via-hole and the pattern is wound counterclockwise. When current is applied to the patterns including the first layer to the fourth layer, the same direction current flows in the pattern of each layer, and an inductance value can be obtained wherein the coil inductances of the respective layer patterns are added.
Accordingly, it is possible to form a coil having a desired inductance value on a printed circuit board. Even when a coil needs to be formed on a circuit, it is unnecessary to attach a coil component. In addition, since the coil is formed throughout the multi-layers of the printed circuit board, there is no case that the area of the board becomes excessively large.
As with JP-A-5-183274, JP-A-2001-77538 discloses a pattern coil on a printed board capable of obtaining a necessary reactance value, a capacitance value, or a resistance value on a board pattern, without mounting discrete coil components, capacitor components, or resistor components. For this reason, C-shaped patterns are formed on the surface of each layer of a build-up multilayer board, and the patterns are coupled to each other by build-up vias, so as to form a coil which is shaped into spiral as a whole. Therefore, a reactance value can be obtained in the board patters.
Since the coil is formed in the board without mounting the discrete coil components or the like, it is possible to reduce the number of components, thereby freely setting coil characteristics (reactance value).
Meanwhile, since magnetic field is generated in the wiring board by providing the wiring board with the inductor, it is necessary to provide a shield to shield and prevent various kinds of components mounted on the wiring board from the magnetic field. For this reason, each circuit block is shielded by separately providing the wiring board with a member only for shield or by providing a metal plate or a shield plate formed of a resin plate, wherein metal plating is applied to a surface of the shield plate, on a circuit board.
FIGS. 2 to 4 show one example in the related art, where a wiring board is provided with a shield plate (panel) for shielding. FIG. 2 is a plan view illustrating a circuit board having a shield pattern formed thereon. FIG. 3 is a plan view illustrating a shield plate (panel) formed of a metal plate or the like. FIG. 4 is a cross-sectional view illustrating a contact portion between a pattern protrusion portion of the shield plate and a shield pattern formed on the wiring board.
As shown in FIG. 2, various kinds of electronic components 11 such as semiconductor elements are mounted on the surface of a wiring board 10. The electronic components include board-mounted inductors 12. A conductive shield pattern 14 is formed on the surface of the wiring board 10 to surround each circumference of circuit blocks 13a to 13f. In FIG. 2, for convenience, an electronic component 11 mounted in one circuit block 13a is schematically shown, and electronic components mounted in the other circuit blocks 13b to 13f are not shown.
As shown in FIG. 3, for example, for each of the blocks 21a to 21f corresponding to the circuit blocks 13a to 13f of the wiring board 10, a shield panel 20 formed of conductive metal plate has a convex pattern 22 corresponding to the conductive shield pattern 14 of the wiring board 10, surrounding circumference thereof. The shield panel 20 is bonded to the wiring board 10, and the shield pattern 14 of the wiring board 10 comes into contact with the convex pattern 22 of the shield panel 20. Accordingly, the wiring board 10 is shielded by the shield panel 20 for each circuit block 13.
As described above, in the related-art inductor built-in wiring board disclosed in JP-A-5-183274 and JP-A-2001-77538, it is possible to form a coil having a desired inductance value on a printed wiring board. However, forming a coil on a printed wiring board, it is difficult to obtain a reasonably satisfactory inductor in light of a pattern forming area, an inductance value, a resonance frequency.
That is, in the inductor built-in wiring board disclosed in JP-A-5-183274 and JP-A-2001-77538, an area occupied by the inductor on the wiring board increases. As a result, a wiring density of the wiring board decreases, or a mounting density of the wiring board including other components decreases.